Here are a couple of suggestions that will help make your hike more enjoyable. Trust me with this and take it very seriously. I am sorry to say that I've seen quite a few people get very ill at high altitude or suffer from cold weather or blisters due to poor preparation. Proper preparation will help you avoid this.
1) Acclimatize first! The hike is strenuous and with your backpack even more so. Try and spend a few days hiking around Cusco first to help you acclimatize (see my Cusco journal for hiking suggestions). I recommend aclimatizing for at least three days. A week is better. If you are reading this upon your arrival in Cusco, you surely have already noticed that walking 10 steps up gives you a solid workout. Imagine walking a couple of thousand steps!
2)Be honest with yourself. Are you reasonably fit? The trail is steep in places, climbing up to a maximum height of 4200m. You don´t need to be a super athlete, but you do need to be in reasonable shape. If you barre not, consider starting at Km 104 for a much less strenuous but still wonderful hike.
3) Bring the right equipment. Shoes are especially important. I've seen many people try the trail in street or tennis shoes, only to end up walking BAREFOOT! Ouch. Bring comfortable hiking shoes and socks. Also, be prepared for pouring rain and cold nights. See my equipment checklist for ideas on what to bring.
4) Safety is in numbers. Hike the trail and camp with a group of people.